This invention relates to an outer rotor type brushless motor having a rotor rotating outside of a stator.
As shown in FIGS. 12, 13A, and 13B, an outer rotor type brushless motor of the prior art comprises an outer rotor 3 having a plural of permanent magnets 2 fixed onto an inner periphery of a cup-like metal rotor yoke 1, a stator 7 including a stator core 4 having a plural of magnetic pole portions 4a protruded on an outer periphery of an annular portion 4b and coils 6 wound on the magnetic pole portions 4a of the stator core 4 and faced to the permanent magnets 2 of the outer rotor 3, respectively, with coil insulation layers 5 provided between the magnetic pole portions 4a and the coils 6, respectively, a cylindrical metal boss 8xe2x80x2 disposed on an inner periphery of the annular portion 4b of the stator core 4, a rotational shaft 10 extending along an axis of the boss 8xe2x80x2 and rotationally supported on the boss 8 by a bearing 9 with a leading end of the shaft 10 having a center portion of the rotor yoke 1 fixed thereto, a metal mounting plate 11 fixed onto an outer periphery of the boss 8xe2x80x2 for attaching the stator 7 on a support body not shown and a circuit board 13 fixed to the mounting plate 11 by screws 12 and having a drive circuit mounted thereon. The annular portions 4b of the stator core 4 are fixed to the outer periphery of the boss 8xe2x80x2 by screws 14. The mounting plate 11 is fixed to the boss 8xe2x80x2 by a pressurization enlarged portion 8xe2x80x2ak formed by deforming an annular base 8xe2x80x2a of the boss 8xe2x80x2 so as to enlarge the diameter thereof after the mounting plate 11 engages the annular base 8xe2x80x2a extending through a hole 15 in the mounting plate 11.
However, since this prior outer rotor type brushless motor has the stator 7 fixed to the boss 8xe2x80x2, which is in turn fixed to the mounting plate 11, the heat of the stator 7 radiates through the metal boss 8xe2x80x2 from the mounting plate 11. Thus, there is a problem that the heat radiation efficiency is poor because of the boss 8xe2x80x2 provided between the stator 7 and the mounting plate 11.
Furthermore, since the outer rotor type brushless motor has the stator core 4 fixed to the boss 8xe2x80x2 by the screws 14 which extend through the annular portion 4b of the stator core 4, there tends to be a gap formed between the annular portion 4b of the stator core 4 and the boss 8xe2x80x2. Thus, there is another problem that it is hard to get the accuracy of assembly after the stator core 4 is fixed to the boss 8xe2x80x2.
Since the step of forming the boss 8xe2x80x2 is performed separately from the step of forming the coil insulation layers 5 on the magnetic pole portions 4a of the stator core 4, much trouble is taken for manufacturing the motor, which disadvantageously causes the cost for manufacture to be higher.
Accordingly, it is an object of the invention to provide an outer rotor type brushless motor adapted to improve the heat radiation characteristic of a stator.
It is another object of the invention to provide an outer rotor brushless motor adapted to be easily assembled while the heat radiation characteristic is improved.
It is further object of the invention to provide an outer rotor type brushless motor adapted to improve the accuracy with which a stator is assembled to a boss while the heat radiation characteristic of the stator is improved.
It is further object of the invention to provide an outer rotor type brushless motor adapted to reduce the cost of manufacture while the accuracy with which a stator is assembled to a boss as well as the heat radiation characteristic of a stator are improved.
The invention is applied to an outer rotor type brushless motor comprising an outer rotor having permanent magnets fixed onto an inner periphery of a cup-like rotor yoke, a stator including a stator core having a plural of magnetic pole portions protruded on an outer periphery of an annular portion of the stator core and faced to the permanent magnets and coils wound on the magnetic pole portions, respectively, a cylindrical boss disposed on an inner periphery of the annular portion of the stator core, a rotational shaft extending along an axis of the boss and rotationally supported on the boss by a bearing with a leading end of the shaft having a center portion of the rotor yoke fixed thereto and a mounting plate fixed onto an outer periphery of the boss.
In the invention, the boss is formed of resin mold and extends through a hole in the mounting plate and fixed thereto. The annular portion of the stator core is fixed to a rising portion of the inner periphery of the mounting plate.
As the annular portion of the stator core is fixed directly to the mounting plate, the heat radiation characteristic of the stator can be improved. As the annular portion of the stator is fixed to the rising portion of the mounting plate, the stator can be fixed without any obstacle such as the coils because of the presence of the rising portion even though the stator is fixed directly to the mounting plate.
In a preferred mode of the invention, the boss may be formed of resin mold, the annular portion of the stator core is molded in and fixed to the outer periphery of the boss and also fixed to the rising portion of the inner periphery of the mounting plate.
As the annular portion of the stator core is fixed directly to the mounting plate, the heat radiation characteristic of the stator can be improved. Also, as the stator core is fixed to the rising portion of the mounting plate, the stator can be fixed without any obstacle such as the coils because of the presence of the rising portion even though the stator is fixed directly to the mounting plate. Furthermore, as the annular portion of the stator core is molded in and fixed to the outer periphery of the boss, there is no gap between the annular portion of the stator core and the boss while the relative position of them is fixed, which improves the accuracy with which the stator is assembled with the boss.